1. Field of the Invention
The present invention relates to a method for fabricating a supermirror for use in neutron guides and neutron mirrors, and more particularly, to a method for fabricating a neutron supermirror, which is widely used in the fabrication of thin films in cold neutron guides and the spectrometer field. In the inventive method, the neutron supermirror is fabricated with nickel thin films and titanium thin films having varying thickness, using a combination of monochromator structures in each of which nickel thin films and titanium thin films, having the same thickness, are alternately stacked on each other. In other words, the supermirror is fabricated with a combination of monochromator structures having a variety of different thicknesses, which show the overlap of diffraction peaks.
2. Description of the Prior Art
Generally, neutrons have positive (+) scattering-length density in most elements, except for some elements such as gadolinium (Gd) and manganese (Mn).
This means that, unlike electromagnetic waves in the visible light spectrum, the incident angle between the neutrons and the plane of the material is large, and neutrons and X-rays can be totally reflected when they are incident on the surface of material at angles smaller than a critical angle.
The refractive index of most materials on which neutrons are incident is n<1, which means that neutrons are totally reflected when they are incident at angles smaller than a critical angle. Those that reflect incident neutrons using this property are called neutron mirrors, and tubes made by accurately joining these neutron mirrors with an error of less than 10 μm are called neutron guides.
Recently, prior nickel neutron guide tubes having a single-layer thin-film structure have been substituted with supermirrors having a multilayer thin-film structure, which can increase neutron yield 2-4 times or more.
The refractive index of neutrons incident on materials can be expressed by the following equation:
  n  =      1    -                            λ          2                          2          ⁢          π                    ⁢                        N          at                ⁡                  (                                    b              c                        ±            p                    )                    wherein λ represents wavelength, bc represents scattering length, Na represents atomic density, and P is g×s, where s represents magnetic moment/atom, and g represents 0.27×10−12 cm/Bohr.
According to the above equation, neutrons incident at angles smaller than the critical angle will be totally reflected.
As shown in FIGS. 1 and 2, a neutron guide 100 is a tube on which nickel or a supermirror is deposited in order to transport cold neutrons from a cold neutron source to an experimental station located a specific distance therefrom in a vacuum without loss.
The neutron guide 100 has a total length of about 40-100 m and is formed by linking a plurality of guide units 110 with each other in series to the desired length.
In each of the guide units 110 constituting the neutron guide 100, pluralities of supermirror substrates 112, each having a length of about 1 m and a very high refractive index, are assembled with each other in a box shape having a rectangular cross section.
Thus, each of such guide units 110 consists of a structure in which neutrons are totally reflected in the guide units at angles smaller than the critical angles due to the supermirror substrates 112 deposited in a thin film.
Meanwhile, it is preferable that the guide units 110 be precisely linked to the guide units 110 on the front and rear sides thereof, such that the size error and alignment error of the front and rear inlets/outlets thereof are maintained at less than 10 μm in order to minimize the loss of neutrons, which can occur during the transport of the neutrons.
In order to transport neutrons to an experimental station without loss, the neutron guide, to which the supermirror for increasing the yield of neutrons is applied, is fabricated by alternately depositing nickel (Ni) and titanium (Ti), so that the total reflection angle (critical angle) thereof can be increased to at least two times that of a neutron mirror fabricated by coating nickel, which reflects neutrons.
This periodical structure of crystal planes diffracts neutrons, electrons, x-rays and the like, and is also used to extract one wavelength.
Such supermirrors include Ni/Ti non-magnetic supermirrors, fabricated by alternating a nickel layer with a titanium layer instead of using a prior Ni single film, and polarizing supermirrors, fabricated using a FeCo/Si, FeCoV/TiZr, Co/Cu or FeCo/Ge film for producing polarized neutrons.
FIG. 3 schematically shows the principle of a monochromator in general use, FIG. 4 schematically shows a TEM photograph of a supermirror in general use, and FIG. 5 is a graphic diagram schematically showing reflectivity as a function of incident angle in a monochromator and supermirror in general, and shows the principle of a Ni/Ti monochromator and a Ni/Ti supermirror, which are in general use.
As shown in the figures, neutrons incident on an interface will be totally reflected when the incident angle (θi) is smaller than the critical angle, and will be transmitted when the incident angle (θi) is larger than the critical angle.
Specifically, unlike visible rays, neutrons (except for some materials, for example, Ti, Mn, Gd, H, V, and Li) or X-rays have a refractive index less than 1, and thus incident neutrons or x-rays will be totally reflected when the incident angle thereof is smaller than the critical angle.
As used herein, the term “reflectivity” refers to the ratio of the absolute intensity of reflected light to that of incident light, which has a value ranging from a maximum of 1 (total reflection) to zero (0).
Herein, the thin film formed by alternately depositing two different materials to a given thickness will produce a diffracted beam, which is called “Bragg Peak”. The monochromator uses the wavelength where this Bragg Peak occurs.
Also, this principle of the monochromator can be used to fabricate a supermirror.
FIGS. 6 and 7 schematically show the principle of polarizing supermirrors in general use.
As shown in the figures, when two different thin films having greatly different scattering length densities are alternately deposited while the thickness thereof is changed, the angle at which the Bragg Peak occurs will change depending on the thickness of the thin films, and the Bragg Peak can continuously occur as a result of adjusting the interval of the change in the thin film thickness. Thus, this principle can be used to fabricate supermirrors.
As shown in FIG. 6, which shows the scattering length densities of materials, a polarizing neutron supermirror is made using materials having different scattering length densities (SLD) of neutrons aligned in the spin-up and spin-down directions under the magnetic field.
For example, when the content ratio of iron (Fe) to cobalt (Co) is set to 89:11, the scattering length density of spin-down neutrons in the iron-cobalt (FeCo) alloy will be the same as the scattering length density of silicon (Si). Thus, when a film of the iron-cobalt (FeCo) alloy and a film of silicon (Si) overlap each other, the spin-down neutrons will be transmitted through both the iron-cobalt (FeCo) alloy film and the silicon (Si) film without distinction, but the spin-up neutrons will be reflected due to the Bragg diffraction phenomenon, as shown in FIG. 7, which shows the path of polarized neutrons.
On the contrary, cobalt (Co) and copper (Cu) have scattering length densities as shown in FIG. 6, and thus the spin-up neutrons will be transmitted through a film of cobalt (Co) and a film of copper (Cu) without distinction, but the spin-down neutrons will be reflected at the interface between cobalt (Co) and copper (Cu).
As described above, the polarizing neutron supermirror can be fabricated by either overlapping the iron-cobalt (FeCo) alloy film with the silicon (Si) film or overlapping the cobalt (Co) film with the copper (Cu) film, and this principle can also be applied either to films of an iron-cobalt-vanadium (FeCoV) alloy film and a titanium-zirconium (TiZr) alloy or to films of iron-cobalt (FeCo) and geranium (Ge) in the same manner.
Various materials can be used to fabricate supermirrors, but generally, a material having a high refractive index and a material having a low refractive index are selectively used. These days, nickel (Ni) and titanium (Ti), which have different refractive indexes, are most widely used.
Herein, assuming that the thickness of one period of nickel (Ni) and titanium (Ti) is d(i), the thickness of a supermirror is calculated according to the following equation:
      d    ⁡          (      i      )        =            d      c              i      4      whereindc: critical thickness=√{square root over (π/ρ( b±p))}/2and b: scattering length density,whereinp=g s,whereing:0.270×10−12 cm/Bohr magnetonand s=M/ρ: average magneticmoment per atoms.
Supermirrors have been fabricated according to the above-described equation, but they had low reflectivity, and for this reason, modified equations have been proposed. Such modified equations have problems in that, because the thickness of thin films for forming supermirrors gradually changes, it is required to precisely control the thickness of the thin films when applying the thin films, and it is difficult to perform an operation of coating an additional thin film on a specific region to increase reflectivity according to the high incident angle.